1. Field of the Invention
The present invention relates to a grinding method and a grinding apparatus of a disk-shaped substrate such as a glass substrate for a magnetic recording medium, and especially relates to a grinding method and a grinding apparatus for grinding an outer circumference and an inner circumference of a disk-shaped substrate.
2. Description of the Related Art
In recent years, the production of disk substrates as disk-shaped substrates has been activated, under increased demands as recording media. As a magnetic disk substrate as one of the disk substrates, an aluminum substrate and a glass substrate are widely used. The aluminum substrate is characterized by its high processability and low cost, meanwhile the glass substrate is characterized by its excellent strength, surface smoothness, and flatness. In particular, requirements for compact size and high density of disk substrates recently have become extremely high, and the glass substrate of which surface roughness is small and that enables high density has attracted a lot of attention.
Various improvements have been made in a manufacturing apparatus of magnetic disk substrates. As the related art described in official gazettes, there is an art of grinding the outer and inner circumferential surfaces of a disk-shaped substrate (a glass substrate, a glass disk) including a portion having a hole at the center (for example, refer to Patent Documents 1 and 2).
In the Patent Document 1, in an inner and outer circumferential surface grinding work apparatus of a glass disk, a related art that performs plural processes in parallel at the same time is disclosed. In the art, a grinding stone for working the outer circumferential surface and a grinding stone for working the inner circumferential surface are displaced with respect to a glass disk fixed to a turn table so as to be brought into contact with the outer circumferential surface and the inner circumferential surface of the glass disk for performing the outer circumferential surface work and inner circumferential surface work in parallel at the same time.
In Patent Document 2, an edge face and a slant face of an outer circumferential portion and an inner circumferential portion of a glass substrate for a hard disk are ground at the same time by a metal bond outer surface grinding stone and a metal bond inner surface grinding stone.
Moreover, the metal bond outer surface grinding stone and the metal bond inner surface grinding stone have plural trapezoid grooves (ten grooves) provided on the same axis with a predetermined interval, in which a half of the ten trapezoid grooves are molded for rough working and the remaining trapezoid grooves for finishing. Further, the edge face and the slant face of the outer circumferential portion of the glass substrate are worked at the same time by the metal bond outer surface grinding stone, while the edge face and the slant face of the inner circumference portion on the glass substrate are worked at the same time by the metal bond inner surface grinding stone.
[Patent Document 1]                Japanese Unexamined Patent Application Publication No. 2005-14176.        
[Patent Document 2]                Japanese Unexamined Patent Application Publication No. 2001-105292.        
As mentioned above, there has been known such an art where an inner circumferential surface (inner circumference) and an outer circumferential surface (outer circumference) of a disk-shaped substrate are ground at the same time. However, grinding contents including work rates are different between grinding on the outer circumference and grinding on the inner circumference such that contact by the grinding stone is in the point contact state on the outer circumference as compared with the inner circumference and a distance to be ground (distance in the circumferential direction) is larger on the outer circumference than on the inner circumference, and moreover, there are a difference in a load on a grinding stone shaft, a difference in circumferential velocity between inner and outer circumferences of a disk-shaped substrate and the like. Further, even when the grinding contents are different as above, high dimensional accuracy and high concentricity are in demand both in the inner and outer circumferences after grinding. Thus, when the inner and outer circumferences of a disk-shaped substrate are ground at the same time, favorable grinding results may not be obtained unless more appropriate grinding conditions are determined.
The present invention is made in order to address the above technical problem and has an object to improve the concentricity of the inner and outer circumferences after grinding in outer circumferential grinding and inner circumferential grinding of a disk-shaped substrate.
Another object of the present invention is to reduce time required for work and to maintain high dimensional accuracy of the inner and outer circumferences after grinding in the outer and inner circumferential grinding of a disk-shaped substrate.